Systems and methods for generating electrosurgical energy using a multistage power converter

ABSTRACT

The electrosurgical systems and methods according to the present disclosure use a multi-stage power converter for generating electrosurgical energy. The electrosurgical systems include an electrosurgical generator having a power converter coupled to an electrical energy source and configured to generate electrosurgical energy. The power converter includes a boost converter configured to convert a first direct current from the electrical energy source to a second direct current, and a phase-shifted pulse width modulation (PS-PWM) resonant inverter configured to invert the second direct current to an alternating current. The electrosurgical generator also includes a plurality of sensors configured to sense a voltage and a current of the generated electrosurgical energy and a controller coupled to the power converter and the plurality of sensors. The controller includes a signal processor configured to determine tissue impedance based on the sensed voltage and current, and an output controller configured to select one of a plurality of output characteristics based on the determined tissue impedance, and to generate control signals to control the boost converter and the PS-PWM resonant inverter, according to the selected output characteristic.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of and priority to U.S. Provisional Application No. 61/858,037, filed on Jul. 24, 2013, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to electrosurgery. More particularly, the present disclosure relates to electrosurgical generators and methods that use a multi-stage power converter for generating electrosurgical energy.

2. Background of Related Art

Electrosurgery involves the application of high-frequency electric current to cut or modify biological tissue during an electrosurgical procedure. Electrosurgery is performed using an electrosurgical generator, an active electrode, and a return electrode. The electrosurgical generator (also referred to as a power supply or waveform generator) generates an alternating current (AC), which is applied to a patient's tissue through the active electrode and is returned to the electrosurgical generator through the return electrode. The alternating current typically has a frequency above 100 kilohertz (kHz) to avoid muscle and/or nerve stimulation.

During electrosurgery, the AC generated by the electrosurgical generator is conducted through tissue disposed between the active and return electrodes. The tissue's impedance converts the electrical energy (also referred to as electrosurgical energy) associated with the AC into heat, which causes the tissue temperature to rise. The electrosurgical generator controls the heating of the tissue by controlling the electric power (i.e., electrical energy per time) provided to the tissue. Although many other variables affect the total heating of the tissue, increased current density usually leads to increased heating. The electrosurgical energy is typically used for cutting, dissecting, ablating, coagulating, and/or sealing tissue.

The two basic types of electrosurgery employed are monopolar and bipolar electrosurgery. Both of these types of electrosurgery use an active electrode and a return electrode. In bipolar electrosurgery, the surgical instrument includes an active electrode and a return electrode on the same instrument or in very close proximity to one another, which cause current to flow through a small amount of tissue. In monopolar electrosurgery, the return electrode is located elsewhere on the patient's body and is typically not a part of the electrosurgical instrument itself. In monopolar electrosurgery, the return electrode is part of a device typically referred to as a return pad.

As shown in FIG. 4, the ideal output power characteristic of an electrosurgical (ES) generator is constant current 410, followed by constant power 420, which is, in turn, followed by constant voltage 430 as a function of the increasing output impedance. To achieve this output power characteristic, the ES generator executes a control loop that samples the output voltage and current, calculates power and/or impedance based on the sampled output voltage and current, feeds the calculated power and/or impedance through a digital compensator, and then adjusts a control signal (e.g., adjusts the pulse width or phase shift of the control signal) that controls the power stage. This control loop, however, may be relatively slow compared to the switching frequency of the power stage. Therefore, the power stage may under-deliver or over-deliver the desired power to the tissue until the control loop catches up with the power stage and the system reaches steady state. For this and other reasons, current ES generators may fall short of providing the ideal output power characteristic.

SUMMARY

The electrosurgical systems and methods of the present disclosure improve the dynamic response of the inverter during power control and reduce the power deviations due to changes in tissue impedances, which reduce thermal spreading in tissue for a given power. The electrosurgical systems and methods of the present disclosure employ a two-staged power converter that provides a desired power level based on the impedance of tissue being treated during an electrosurgical procedure. The two-staged power converter includes a boost converter and a phase-shifted pulse width modulation (PS-PWM) resonant inverter. The boost converter converts input direct current to a desired direct current and the PS-PWM resonant inverter inverts the desired direct current to a desired alternating current suitable for a given electrosurgical procedure.

The boost converter is controlled by control signals generated based on a current programmed mode or a voltage control mode and a current programmed mode. The PS-PWM resonant inverter is controlled by control signals having a desired fixed phase. The control signals that are used to control the boost converter and the PS-PWM resonant inverter are determined based on the output characteristic, e.g., based on whether the output characteristic is constant current, constant power, or constant voltage.

In one aspect, the present disclosure features an electrosurgical generator that includes an power converter, a plurality of sensors, and a controller. The power converter is coupled to an electrical energy source and generates electrosurgical energy. The power converter includes a boost converter that converts a first direct current from the electrical energy source to a second direct current and a phase-shifted pulse width modulation (PS-PWM) resonant inverter that inverts the second direct current to an alternating current. The plurality of sensors sense voltage and current waveforms of the generated electrosurgical energy. The controller is coupled to the power converter and the plurality of sensors, and includes a signal processor and an output controller. The signal processor determines tissue impedance based on the voltage and current waveforms. The output controller selects one among a plurality of output characteristics based on the determined tissue impedance and generates a first control signal to control the boost converter and a second control signal to control the PS-PWM resonant inverter, according to the selected output characteristic.

The plurality of output characteristics may include a constant current output characteristic, a constant power output characteristic, and a constant voltage output characteristic. The output controller may shift from the constant current output characteristic to the constant power output characteristic and from the constant power output characteristic the constant voltage output characteristic based on the tissue impedance. The output controller may select the constant current output characteristic if the tissue impedance is less than a first predetermined value, the output controller may select the constant power output characteristic if the tissue impedance is greater than or equal to the first predetermined value and less than a second predetermined value, the output controller may select the constant voltage output characteristic if the tissue impedance is greater than or equal to the second predetermined value and less than a third predetermined value, and the first predetermined value may be less than the second predetermined value and the second predetermined value may be less than the third predetermined value.

When the output characteristic is a constant voltage output characteristic, the output controller may generate the first control signal under a voltage control mode and may generate the second control signal having a fixed phase. When the output characteristic is a constant current output characteristic, the output controller may generate the first control signal under a voltage control mode and may generate the second control signal having a fixed phase. When the output characteristic is a constant power output characteristic, the output controller may generate the first control signal under a current programmed mode and may generate the second control signal having a fixed phase. Alternatively, when the output characteristic is any one of a constant voltage output characteristic, a constant current output characteristic, and a constant power output characteristic, the output controller may generate the first control signal under a current programmed mode and may generate the second control signal having a fixed phase.

The output controller may operate the boost converter at a faster switching frequency than a switching frequency of the PS-PWM resonant inverter. The boost converter may include a plurality of boost converters to lower ripples of the voltage and current input to the PS-PWM resonant inverter.

The electrosurgical generator may further include analog-to-digital converters (ADCs) that sample the sensed voltage and current waveforms to obtain a predetermined number of samples of the sensed voltage and current waveforms. The predetermined number of samples may correspond to an integer multiple of an RF frequency of the sensed voltage and current waveforms. The signal processor may include a plurality of ADC controllers that provide control parameters to the ADCs. The control parameters may include a sampling frequency of the ADCs.

The plurality of sensors may include a Rogowski coil. The controller of the electrosurgical generator may be implemented by a field programmable gate array, an application specific integrated circuit, a digital signal processor, or a programmable digital signal processor.

The present disclosure, in another aspect, features a method for controlling an electrosurgical generator. The method includes converting a first direct current from an electrical energy source to a second direct current using a boost converter, converting the second direct current to an alternating current using a PS-PWM inverter, sensing a current of the boost converter and a voltage at an output of the PS-PWM inverter, determining an impedance of tissue being treated based on the sensed voltage and current waveforms, selecting a output characteristic based on the determined tissue impedance, and generating a first control signal to control the boost converter and a second control signal to control the PS-PWM inverter, according to a predetermined control mode for the selected output characteristic. The plurality of output characteristics may include a constant current output characteristic, a constant voltage output characteristic, and a constant power output characteristic.

When the output characteristic is a constant voltage output characteristic, the first control signal may be generated under a voltage control mode and the second control signal may be generated to have a fixed phase. When the output characteristic is a constant current output characteristic, the first control signal may be generated under a voltage control mode and the second control signal may be generated to have a fixed phase. When the output characteristic is a constant power output characteristic, the first control signal may be generated under a current programmed mode and the second control signal may be generated to have a fixed phase. Alternatively, when the output characteristic is any one of a constant voltage output characteristic, a constant current output characteristic, and a constant power output characteristic, the first control signal may be generated under a current programmed mode and the second control signal may be generated to have a fixed phase.

BRIEF DESCRIPTION OF THE DRWAINGS

Various embodiment of the present disclosure are described with reference to the accompanying drawings wherein:

FIG. 1 is an illustration of an electrosurgical system in accordance with embodiments of the present disclosure;

FIG. 2 is a circuit block diagram illustrating the generator circuitry of the electrosurgical generator of FIG. 1;

FIG. 3 is a circuit block diagram of a controller of the generator circuitry of FIG. 2;

FIG. 4 is a graphical diagram of the ideal output characteristics of the electrosurgical generator of FIG. 1;

FIG. 5 is a graphical diagram of power transfer from the electrosurgical generator of FIG. 1 to tissue;

FIG. 6 is a circuit block diagram of generator circuitry according to embodiments of the present disclosure;

FIG. 7A is a circuit block diagram of generator circuitry according to another embodiment of the present disclosure;

FIG. 7B is a circuit block diagram of a current programmed mode (CPM) controller of the generator circuitry of FIG. 7A;

FIG. 7C is a circuit block diagram of a voltage mode controller of the generator circuitry of FIG. 7A;

FIG. 8 is a circuit block diagram of generator circuitry according to still another embodiment of the present disclosure;

FIG. 9 is a flow diagram illustrating a method of controlling an output power converter according to an embodiment of the present disclosure; and

FIG. 10 is a flow diagram illustrating a method of controlling an output power converter according to another embodiment of the present disclosure.

DETAILED DESCRIPTION

To select an output characteristic among the constant current, constant voltage, and constant power output characteristics, typical methods sample output voltage and current waveforms, calculate power and/or impedance, feed these calculation results through a digital compensator, and then adjust the control variables of the output power converter's buck converter and resonant inverter. These processes are relatively slow compared to the switching frequency of the output power converter and, therefore, over-deliver or under-deliver the desired power until the control processes of the output power converter catch up and the system reaches a steady state. Thus, it is desirable to switch among the constant current, the constant power, and the constant voltage output characteristics quickly.

The systems and methods according to the present disclosure employ a multi-stage output power converter that can achieve a near ideal constant-current, constant-power, and constant-voltage output characteristic by changing the control methodology of each stage. The multi-stage output power converter may be a dual-stage output power converter that includes a buck/boost converter and a resonant inverter that are separately controlled according to selected control modes. The control modes are selected based on the desired output characteristic, that is, constant current, constant voltage, or constant power. The desired output characteristic, in turn, is selected based on the measured tissue impedance. In this manner, the systems and methods of the present disclosure provide a desired amount of power and switch between output characteristics more quickly.

The control methods according to the present disclosure may be implemented in hardware and firmware. Because of the improved control loop bandwidth that is achieved by the hardware according to the present disclosure, the firmware control may be simplified and updated at a slower rate than the resonant inverter output frequency.

FIG. 1 illustrates an electrosurgical system 100 in accordance with embodiments of the present disclosure. The electrosurgical system 100 includes an electrosurgical generator 102 which generates electrosurgical energy to treat tissue of a patient. The electrosurgical generator 102 generates an appropriate level of electrosurgical energy based on the selected mode of operation (e.g., cutting, coagulating, ablating, or sealing) and/or the sensed voltage and current waveforms of the generated electrosurgical energy. The electrosurgical system 100 may also include a plurality of output connectors corresponding to a variety of electrosurgical instruments.

The electrosurgical system 100 further includes a monopolar electrosurgical instrument 110 having an electrode for treating tissue of the patient (e.g., an electrosurgical cutting probe or ablation electrode) with a return pad 120. The monopolar electrosurgical instrument 110 can be connected to the electrosurgical generator 102 via one of the plurality of output connectors. The electrosurgical generator 102 may generate electrosurgical energy in the form of radio frequency (RF) energy. The electrosurgical energy is supplied to the monopolar electrosurgical instrument 110, which applies the electrosurgical energy to tissue. The electrosurgical energy is returned to the electrosurgical generator 102 through the return pad 120. The return pad 120 provides sufficient contact area with the patient's tissue so as to minimize the risk of tissue damage due to the electrosurgical energy applied to the tissue.

The electrosurgical system 100 also includes a bipolar electrosurgical instrument 130. The bipolar electrosurgical instrument 130 can be connected to the electrosurgical generator 102 via one of the plurality of output connectors. Alternating current is supplied to one of the two forceps, is applied to tissue, and is returned to the electrosurgical generator 102 through the other forceps.

The electrosurgical generator 102 may be any suitable type of generator and may include a plurality of connectors to accommodate various types of electrosurgical instruments (e.g., monopolar electrosurgical instrument 110 and bipolar electrosurgical instrument 130). The electrosurgical generator 102 may also be configured to operate in a variety of modes, such as ablation, cutting, coagulation, and sealing. The electrosurgical generator 102 may include a switching mechanism (e.g., relays) to switch the supply of RF energy among the connectors to which various electrosurgical instruments may be connected. For example, when an electrosurgical instrument 110 is connected to the electrosurgical generator 102, the switching mechanism switches the supply of RF energy to the monopolar plug. In embodiments, the electrosurgical generator 102 may be configured to provide RF energy to a plurality of instruments simultaneously.

The electrosurgical generator 102 includes a user interface having suitable user controls (e.g., buttons, activators, switches, or touch screens) for providing control parameters to the electrosurgical generator 102. These controls allow the user to adjust parameters of the electrosurgical energy (e.g., the power level or the shape of the output waveform) so that the electrosurgical energy is suitable for a particular surgical procedure (e.g., coagulating, ablating, tissue sealing, or cutting). The electrosurgical instruments 110 and 130 may also include a plurality of user controls. In addition, the electrosurgical generator 102 may include one or more display screens for displaying a variety of information related to the operation of the electrosurgical generator 102 (e.g., intensity settings and treatment complete indicators).

FIG. 2 is a circuit block diagram of generator circuitry 200 within the electrosurgical generator 102 of FIG. 1. The generator circuitry 200 includes a low frequency (LF) rectifier 220, a radio frequency (RF) amplifier 230, a plurality of sensors 235 and 240, analog-to-digital converters (ADCs) 250, a controller 260, which includes a hardware accelerator 270 and a processor subsystem 280, and a user interface (UI) 290. The generator circuitry 200 is configured to connect to an alternating current (AC) power source 210, such as a wall power outlet or other power outlet, which generates AC having a low frequency (e.g., 25 Hz, 50 Hz, or 60 Hz). The AC power source 210 provides AC power to the LF rectifier 220, which converts the AC to direct current (DC).

The direct current (DC) output from the LF rectifier 220 is provided to the RF amplifier 230, which includes a converter 232 and a resonant inverter 234. The combination of the converter 232 and the resonant inverter 234 forms a multi-stage power converter described in more detailed below. The converter 232 steps up or steps down the DC to a desired level. The resonant inverter 234 inverts the DC to an AC waveform to treat tissue. The AC waveform has a frequency suitable for an electrosurgical procedure (e.g., 472 kHz, 29.5 kHz, and 19.7 kHz).

The appropriate frequency for the electrosurgical energy may differ based on the electrosurgical procedures and modes of electrosurgery. For example, nerve and muscle stimulations cease at about 100,000 cycles per second (100 kHz) and some electrosurgical procedures can be performed safely at a radio frequency (RF) above 100 kHz. At frequencies over 100 kHz, the electrosurgical energy can pass through a patient to targeted tissue with minimal neuromuscular stimulation. For example, ablation uses a frequency of 472 kHz. Other electrosurgical procedures can be performed at frequencies lower than 100 kHz, e.g., 29.5 kHz or 19.7 kHz, with minimal risk of damaging nerves and muscles. The resonant inverter 234 may output AC signals with various frequencies suitable for electrosurgical operations.

As described above, the RF amplifier 230 includes a resonant inverter 234 which is coupled to the converter 232. The resonant inverter 234 matches the impedance at converter 232 to the impedance of the tissue so that there is maximum or optimal power transfer from the RF amplifier 230 to the tissue being treated.

The electrosurgical energy provided by the converter 232 of the RF amplifier 230 is controlled by the controller 260. The voltage and current waveforms of the electrosurgical energy output from the converter 232 and the resonant inverter 234 are sensed by the plurality of sensors 235, 240 and are provided to the controller 260, which generates control signals to control the output voltage and current waveforms of the converter 232 and the resonant inverter 234. The controller 260 also receives input commands from a user via the user interface (UI) 290. The UI 290 allows a user to select a type of electrosurgical procedure (e.g., monopolar or bipolar) and a mode (e.g., coagulation, ablation, sealing, or cutting), or input desired control parameters for the electrosurgical procedure or the mode. The UI 290 also includes a display (e.g., an LCD display) that displays, among other things, information related to characteristics of the electrosurgical energy (e.g., a selected power level).

The plurality of sensors 235, 240 may include two or more pairs or sets of voltage and current sensors that provide redundant measurements of the voltage and current waveforms. This redundancy ensures the reliability, accuracy, and stability of the voltage and current measurements at the output of the converter 232 and resonant inverter 234. In embodiments, the plurality of sensors 235, 240 may include fewer or more sets of voltage and current sensors depending on the application or the design requirements.

In embodiments, the current passing through the converter 232 is sensed by a current sensor of the plurality of sensors 235 and a voltage of the resonant inverter 234 is sensed by a voltage sensor of the plurality of sensors 240. The plurality of sensors 235, 240 may employ any known technology for measuring voltage and current including, for example, a Rogowski coil.

The sensed voltage and current waveforms are fed to analog-to-digital converters (ADCs) 250, which sample the sensed voltage and current waveforms to obtain digital samples of the voltage and current waveforms. The digital samples of the voltage and current waveforms are processed by the controller 260 and used to generate control signals to control the converter 232 and the resonant inverter 234 of the RF amplifier 230. The ADCs 250 may be configured to sample the sensed voltage and current waveforms at a sample period that is an integer multiple of the RF frequency of the voltage and current waveforms.

As shown in FIG. 2, the controller 260 includes a hardware accelerator 270 and a processor subsystem 280. The hardware accelerator 270 processes the output from the ADCs 250 and cooperates with the processor subsystem 280 to generate control signals for the converter 232 and resonant inverter 234 of the RF amplifier 230.

The hardware accelerator 270 includes a dosage monitoring and control (DMAC) 272, an inner power control loop 274, an inverter controller 276, and a converter controller 278. All or a portion of the controller 260 may be implemented by a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), a digital signal processor (DSP), and/or a microcontroller.

The DMAC 272 receives samples of the voltage and current waveforms from the ADCs 250 and calculates the impedance of the tissue. The DMAC 272 then provides the impedance of the tissue to the inner power control loop 274, which generates control signals for the inverter controller 276 and the converter controller 278 based on the impedance of the tissue. The inverter controller 276, in turn, generates a first control signal to control the output of the resonant inverter 234 and the converter controller 278, in turn, generates a second control signal to control the output of the converter 232. The first and second control signals act to limit the RF amplifier's output voltage and current to a desired voltage and current as specified in a particular control mode. In this manner, the controller 260 controls the RF amplifier 230 to produce near deadbeat control of the output power.

The processor subsystem 280 includes an outer power control loop 282, a state machine 284, and a power setpoint circuit 286. The processor subsystem 280 generates a control signal based on the output of the DMAC 272 and parameters (e.g., electrosurgical mode) selected by the user via the UI 290. Specifically, the parameters selected by the user are provided to the state machine 284 which determines a state or mode of the generator circuitry 200. The outer power control loop 282 uses this state information and the output from the DMAC 272 to determine a control signal. The control signal is provided to the power setpoint circuit 286 which generates a power setpoint value based on the control signal.

The inner power control loop 274 uses the power setpoint value to generate appropriate control signals for controlling the converter 232 via the converter controller 278. If the user does not provide operational parameters to the state machine 284 via the UI 290, then the state machine 284 may maintain or enter a default state.

FIG. 3 is a circuit block diagram of the hardware accelerator 270 of FIG. 2. The hardware accelerator 270 implements functions of the generator circuitry 200 that may have special processing requirements such as high processing speeds. The hardware accelerator 270 includes the DMAC 272 and the inner power control loop 274 shown in FIG. 2.

The DMAC 272 includes four ADC controllers 312 a-312 d, a digital signal processor 314, RF data registers 316, and DMAC registers 318. The ADC controllers 312 a-312 d control the operation of the ADCs 250, which convert sensed voltage and current waveforms into digital data. The digital data is then provided to the digital signal processor 314 that implements various digital signal processing functions, some of which are described in more detail below.

The ADC controllers 312 a-312 d provide operational parameters, including a predetermined sampling rate, to the ADCs 250 so that the ADCs 250 sample the sensed voltage and current waveforms synchronously at a predetermined sampling rate, i.e., a predetermined number of samples per second, or a predetermined sampling period. The ADC controllers 312 a-312 d may be configured to control the ADCs 250 so that the sampling period corresponds to an integer multiple of the RF frequency of the voltage and current waveforms.

The DMAC 272 provides a control signal, which is the impedance of the tissue being treated, to the inner power control loop 274 via signal line 321 and to the processor subsystem 280 via signal line 379. The inner power control loop 274 processes the control signal and outputs a control signal to the inverter controller 276 and the converter controller 278. The inner power control loop 274 includes an inner power controller 326, compensator registers 330, and VI limiter 334.

When there is a user input, the processor subsystem 280 receives the user input and processes it with the outputs from the digital signal processor 314 via a signal line 379. The processor subsystem provides control signals via compensator registers 330 to a VI limiter 334, which corresponds to the power setpoint circuit 286 in FIG. 2. The VI limiter 334 then provides via signal line 335 a desired power profile (e.g., minimum and maximum limits on the power for a selected electrosurgical mode or operation) based on the user input and the output of the digital signal processor 314. The compensator registers 330 also provide via signal line 333 other control parameters to the inner power controller 326, which combines control parameters from the compensator registers 330, the VI limiter 334, and the impedance from the digital signal processor 314 to generate control signals and provide them to the inverter controller 276 via signal line 327 and to the converter controller 278 via signal line 328.

The inverter controller 276 receives a control parameter and outputs control signals that drive the resonant inverter 234. The inverter controller 276 includes a scale unit 342, PWM registers 344, and the PWM module 346. The scale unit 342 scales the output of the compensator registers 330 by multiplying and/or adding a scaling value to the output. The scale unit 342 receives a scaling value from the PWM registers 344 via signal lines 341 a and 341 b. The PWM registers 344 store several relevant parameters to control the resonant inverter 234, e.g., a period, a pulse width, and a phase of the AC signal to be generated by the resonant inverter 234 and other related parameters. The PWM module 346 receives outputs from the PWM registers 344 and generates four control signals 347 a-347 d that control four transistors of the resonant inverter 234 of the RF amplifier 230 of FIG. 2. The PWM module 346 also synchronizes its information with the information in the PWM registers 344 via a register sync signal.

The converter controller 278 receives a control signal and generates another control signal so that the converter 232 is controlled to amplify or step down direct current to a desired level suitable for the resonant inverter 234. The converter controller 278 includes PWM registers 352 and a PWM module 354. The PWM registers 352 receive outputs from the inner power control loop 274 and stores relevant parameters in the PWM registers 344 of the inverter controller 276. The PWM module 354 sends a register sync signal to the PWM registers 352 and generates a control signal 355 having a desired duty cycle to control the converter 232 of FIG. 2. Thus, the inner power control loop 274 controls the converter 232 and the resonant inverter 234 of the RF amplifier 230 based on processed sensor signals provided by the DMAC 272 and other signals provided to the hardware accelerator 270 via the processor subsystem 280.

Current electrosurgical generators employ a variety of output power stages including a high-voltage DC (HVDC) power supply followed by a fixed PWM inverter, a HVDC power supply followed by a current source inverter, a phase-shifted PWM full bridge resonant inverter, a current-source parallel-resonant DC/AC inverter with a transformer, and an IGBT-based LCL-resonant inverter for high-frequency induction heating. These output power stages have an output impedance that affects the transfer of power to the load. As shown in FIG. 5, these output power stages exhibit an output power characteristic of the resonant inverter, in which the power initially increases along an asymptotic line 505 of constant current and then increases more slowly until the output impedance matches the source impedance of the electrosurgical generator. When the output impedance matches the source impedance, maximum power 510 is transferred to the tissue being treated. Then, as the output impedance further increases, the power slowly decreases until the power decreases along an asymptotic line 515 of constant voltage.

As described herein, the systems and methods according to the present disclosure employ a multi-stage output power converter that can achieve a near ideal constant-current, constant-power, and constant-voltage output characteristic by separately selecting the control methodology of each stage of the multi-stage output power converter. FIG. 6 is a circuit block diagram of generator circuitry 600 that may be employed in the electrosurgical generator 102 of FIG. 1. The generator circuitry 600 includes a voltage source 610, a DC-DC boost converter 630, and a phase-shifted (PS) pulse width modulation (PWM) resonant inverter 650.

The voltage source 610 provides direct current i_(g)(t) 620 to the DC-DC boost converter 630, which steps down or steps up the voltage of the direct current to a desired voltage level v₁ 640. Then, the PS-PWM resonant inverter 650 inverts the desired DC voltage v₁ 640 provided by the DC-DC boost converter 630 into AC having a voltage and a frequency suitable for treating tissue. The AC voltage v₀ output from the PS-PWM resonant inverter 650 is then provided to the tissue load Z_(load) 680.

The controller 260 of FIG. 2 may obtain samples of the DC voltage v₁ 640 via sensors 235 and the ADCs 250 and may generate a control signal d 635 to control the DC voltage v₁ 640 based on the samples of the DC voltage v₁ 640. The controller 260 may also obtain samples of the AC current i_(o)(t) 670 and AC voltage v_(o) 660 and may generate another control signal p 655 having a fixed phase to control the PS-PWM resonant inverter 650 based on the samples of the AC current i_(o)(t) 670 and AC voltage v_(o) 660.

FIG. 7A is a circuit block diagram of generator circuitry 700 including a power converter 701 and a controller 715 for controlling the power converter 701. The power converter 701 includes a boost converter 705 and a PS-PWM resonant inverter 707 connected to the output of the boost converter 705. The PS-PWM resonant inverter 707 includes a series-parallel LCLC resonant tank circuit 703. The series-parallel LCLC resonant tank circuit 703 includes an inductor 721 and a capacitor 723 connected in series with the output of the power converter 701, the primary inductor coil of the transformer 727, and a capacitor 725 connected in parallel with the output of the power converter 701.

The controller 715 includes a CPM controller 712 and a voltage mode controller 714 that control the boost converter 705 with a control signal d having a desired duty cycle, and a phase-shifted PWM controller 716 that controls the PS-PWM resonant inverter 707 with a PWM control signal p having a fixed phase. The controller 715 also includes a switch 711 that switches between the CPM controller 712 and the voltage mode controller 714.

The CPM controller 712, the voltage mode controller 714, and the phase-shifted PWM controller 716 operate according to the desired output characteristic as illustrated in Table 1 below. As described above, the desired output characteristic changes from constant current, to constant power, to constant voltage depending upon the output impedance.

TABLE 1 Desired Output Constant Constant Power Constant Characteristic: Current Voltage Control Signal d Voltage mode CPM (current Voltage mode (provided to control programmed mode) control the boost converter 705) Control Signal Fixed phase p₁ Fixed phase Fixed phase p₂ p (provided to the PS-PWM resonant inverter 707)

To achieve a constant current output at the beginning of tissue treatment, the controller 715 switches the switch 711 to the voltage mode controller 714, which generates the control signal d having a fixed duty cycle and provides the control signal d to the switch 708 of the DC-DC boost converter 705.

Additionally, the phase-shifted PWM controller 716 generates a PWM control signal p having a first fixed phase p₁ and provides it to the H-bridge 717 of the DC-AC PS-PWM resonant inverter 707. The phase-shifted PWM controller 716 varies the duty cycle of the PWM control signal p so that the PS-PWM resonant inverter 707 outputs a constant current.

The voltage mode control involves measuring the output voltage v₁ of the DC-DC boost converter 705, feeding the measured output voltage v₁ to the voltage mode controller 714, and adjusting the duty cycle of control signal d based on the difference between the measured output voltage v₁ and a reference output voltage so that the measured output voltage v₁ matches the reference output voltage. The reference output voltage may be set by a user or may be based on reference output voltage values stored in a look-up table. In the voltage control mode, the series impedance 723 of the tank circuit 703 limits the output current.

When the output impedance reaches a first predetermined impedance value, the desired output characteristic changes from constant current to constant power. For the constant power output characteristic, the controller 715 changes the switch 711 to the current programmed mode (CPM) controller 712. The current programmed mode controller 712 varies the duty cycle of the control signal d according to the current programmed mode to maintain a constant power output from the DC-DC boost converter 705. Additionally, the PS-PWM controller 716 generates the control signal p having a fixed phase and a fixed duty cycle, and provides it to the H-bridge 717 of the DC-AC PS-PWM resonant inverter 707.

When the output impedance reaches a second predetermined impedance value, the desired output characteristic changes from constant power to constant voltage. For the constant voltage output characteristic, the controller 715 switches the switch 711 back to the voltage mode controller 714, which varies the duty cycle of the control signal d according to the voltage control mode and provides it to the switch 708 of the DC-DC boost converter 705. In the voltage control mode, the voltage mode controller 714 operates the switch 708 to adjust the voltage to maintain a constant voltage output as the output impedance further changes or increases over time. Additionally, the PS-PWM controller 716 generates the control signal p having a second fixed phase p₂ and a fixed duty cycle. In the voltage control mode, the parallel impedance 725 of the tank circuit 703 naturally limits the output voltage.

In embodiments, to switch the boost converter 705 and the PS-PWM resonant inverter 707 between control methods, the output voltage and current may be measured and compared to voltage thresholds set by the control logic, e.g., an FPGA or DSP. The output voltage and current measurements may be rectified by a rectifier and fed to a comparator. A DAC may be connected to the comparator to provide voltages proportional to voltage and current limits. The output of the comparator would then be used to determine the control method of the boost converter 705 and the PS-PWM resonant inverter 707. For example, when the desired output characteristic is constant current and the comparator determines that the measured output voltage has reached the voltage limit, a switch 711 switches from the voltage mode controller 714 to the CPM controller 712.

The boost converter 705 behaves as a constant power source when it is operated in the current program mode. Thus, when the PS-PWM controller 716 controls the PS-PWM resonant inverter 707 with a fixed phase p from the boost converter 705, it can deliver the ideal voltage limit, current limit, and power limit with near deadbeat control.

FIG. 7B is a circuit block diagram of the CPM controller 712 of the controller of FIG. 7A. The CPM controller 712 includes a summing block 710, a compensator 720, a comparator 740 with an inverting output, a current sense resistor 730, a latch 750, a clock 760, and driver circuitry 765 for driving the switch 708 of the boost converter 705. The summing block 710 receives a desired voltage v_(ref) and an output voltage v₀ from the resonant inverter 650, which is sensed by a voltage sensor, and determines the difference between the output voltage and the desired voltage. The difference between the output voltage and the desired voltage is then provided to the compensator 720. The compensator 720 outputs a compensator voltage to the comparator 740. The compensator voltage is represented by the product i_(c)(t)R_(f), where i_(e)(t) represents a compensator current and Rf represents the resistance of the current sense resistor 730.

The controller 715 senses the switch current i_(s)(t) passing through the switch 708 of the boost converter 705 using the current sense resistor 730. The current sense resistor 730 provides a switch voltage i_(s)(t)R_(f) to the comparator 740. In some embodiments, the CPM controller 712 may further include a summing block connected between the current sense resistor 730 and the comparator 740. The summing block may add an artificial ramp i_(d)(t)R_(f), which may be generated by the clock 760, to the switch voltage i_(s)(t)R_(f). The comparator 740 then compares the compensator voltage i_(c)(t)R_(f) to the switch voltage i_(s)(t)R_(f). If the switch voltage i_(s)(t)R_(f) is less than the compensator voltage i_(c)(t)R_(f), the comparator 740 sets the latch 750 so that a clock signal generated by the clock 760 is provided to the driver circuitry 765 to drive the switch 708 according to the clock signal. If the switch voltage i_(s)(t)R_(f) reaches or exceeds the compensator voltage i_(c)(t)R_(f), the comparator 740 outputs a nonzero digital value to the reset input R of the latch 750 to reset the latch 750. When the latch 750 is reset, the latch 750 outputs a zero value to the driver circuitry 765, which sets the control signal d to zero, thereby turning off the switch 708. In this manner, the compensator current i_(c)(t) acts as a current limit to the switch current i_(s)(t).

FIG. 7C is a circuit block diagram of the voltage mode controller 714 of the generator circuitry 700 of FIG. 7A. The voltage mode controller 714 includes a summing block 745, a compensator 755, and a PWM controller 770. The summing block 745 determines the difference between the feedback voltage v₁ and reference voltage v_(ref), which, for example, may be the desired voltage for the constant voltage output characteristic. The compensator 755 outputs an error signal based on the output from the summing block 745. Then, the PWM controller 770 varies the duty cycle of the driving signal d based on the error signal output from the compensator 755.

FIG. 8 is a circuit block diagram of generator circuitry 800 according to another embodiment in which an output power converter 801 includes one or more boost converters 705 a and 705 b having respective switches 708 a and 708 b, and the resonant inverter 707 of FIG. 7A. The output power converter 801 is controlled by controller 849 which includes CPM controller 712 for controlling the one or more boost converters 705 a and 705 b, respectively, and the PS-PWM controller 716 for controlling the PS-PWM resonant inverter 707.

In an embodiment in which the output power converter 801 includes only one CPM controller, e.g., CPM controller 712, the controller 849 operates the output power converter 801 according to the desired output characteristics as shown in Table 2 below.

TABLE 2 Desired Output Constant Voltage Constant Current Constant Power Characteristic Control Signal d₁ CPM (current CPM (current CPM (current (provided to, programmed programmed programmed e.g., the boost mode) mode) mode) converter 705a) Contro Signal pl Fixed phase Fixed phase Fixed phase (provided to the PS-PWM resonant inverter 707)

As shown in Table 2, the CPM controller 712 may generate a control signal d₁ for the boost converter 705 a according to CPM for all the output characteristics, and the PWM controller 716 generates a control signal p for the PS-PWM resonant inverter 707 having a fixed phase for all the output characteristics. The controller 849 may also run a slow control loop to deliver a desired power dosage. At the voltage/current limits, the boost converter 705 a would try to deliver constant power to the PS-PWM resonant inverter 707 and the PS-PWM resonant inverter 707 would consume the difference in the power delivered to the load and the power supplied by the boost converter 705 a. Essentially, the boost converter 705 a and the PS-PWM resonant inverter 707 are run open loop to deliver the ideal output characteristics.

The boost converter 705 a may be run at a faster switching frequency compared to the PS-PWM resonant inverter 707, e.g., N-times faster. Thus, during the constant power output characteristic, the output of the PS-PWM resonant inverter 707 would deliver constant power over 1/Nth of a cycle.

The output power converter 801 may include two or more boost converters coupled together in parallel and controlled by two or more corresponding control signals that are shifted in time with respect to each other in a symmetrical way. In the case of two boost converters, e.g., boost converters 705 a and 705 b, the control signals for the boost converters, e.g., control signals d₁ and d₂, may have a 180-degree phase difference. In the case of three boost converters, the phase difference between any two of the three corresponding control signals would be 120 degrees. In the case of four boost converters, the phase difference between any two of the four corresponding control signals would be 90 degrees. By increasing the number of boost converters, the generator circuitry 200 may achieve an ideal power versus impedance output characteristic, lower output measurement sampling rates, improved clamping of voltage and current without using active electrical elements, such as an active snubber, and control constant power delivery.

When operating under CPM, each boost converter maintains a constant average power over its own cycle and all the phases deliver an average constant power over their multi-phase period. The output power converter topology using two or more boost converters is inherently slower than the single boost stage running at N times the speed of the PS-PWM resonant inverter 707 because the total average power is only constant over an entire multi-phase period. But, the multi-boost converter topology provides lower input voltage and current ripple than the single boost converter topology.

FIG. 9 is a flow diagram illustrating a method 900 of controlling the output power converter of FIG. 7A based on the impedance of the tissue being treated. In this method, the desired output characteristic is changed from constant current, to constant power, and to constant voltage based on the impedance of the tissue being treated. After starting in step 901, the controller 715 operates the output power converter to provide a constant current output characteristic. Specifically, the controller 715 generates a control signal d according to the voltage mode control and a control signal p having a fixed phase p₁, and provides them to the boost converter 705 and the phase-shifted pulse width modulation (PS-PWM) resonant inverter 707 in steps 902 and 904, respectively. The controller 715 then measures the impedance of the tissue being treated in step 906. The measured impedance is compared to the first predetermined impedance value Z₁ in step 908. If the measured impedance is less than the first predetermined impedance value Z₁, steps 902 and 904 are repeated to provide a constant current output characteristic. Otherwise, steps 910 and 912 are performed to provide a constant power output characteristic.

In step 910, a control signal d is generated according to the current programmed mode and is provided to the boost converter 705. In step 912, a control signal p having a fixed phase is generated and provided to the boost converter 705. The controller 715 then measures the impedance of the tissue being treated in step 914. In step 916, the measured impedance is compared to a second predetermined impedance value Z₂. If the measured impedance is less than the second predetermined impedance value Z₂, the method 900 returns to step 908, and steps 910 and 912 are repeated to provide a constant power output characteristic. Otherwise, steps 918 and 920 are performed to provide a constant voltage output characteristic.

In steps 918 and 920, the controller 715 generates a control signal d according to the voltage mode control and a control signal p having a fixed phase p₂, and provides them to the boost converter 705 and the PS-PWM resonant inverter 707, respectively. The controller 715 then measures the impedance of the tissue being treated in step 922. In step 924, the measured impedance is compared to the third predetermined impedance value Z₃. If the measured impedance is less than the third predetermined impedance value Z₃, the method 900 returns to step 916 and performs steps 918-924 based on the constant voltage output characteristic. Otherwise, the method 900 of controlling the power converter 701 returns to step 922 to continue measuring the output impedance, e.g., the tissue impedance. Since step 922 does not generate control signals, the boost converter and the PS-PWM resonant inverter do not output voltage and current waveforms to the tissue being treated. Nevertheless, the electrosurgical operation does not end until a user turns off a power switch of the electrosurgical generator or terminates supplying power to the electrosurgical generator.

FIG. 10 is a flow diagram illustrating a method 1000 of controlling the output power converter 801 of FIG. 8. As shown in FIG. 8, the controller 849 controls two boost converters 705 a and 705 b and the PS-PWM resonant inverter 707. After starting in step 1001, the controller 849 generates, in step 1002, two control signals d₁ and d₂ having different phases according to the current programmed mode and provides them to the two boost converters to control the boost converters 705 a and 705 b, respectively. In step 1004, the controller 849 generates a control signal p having a fixed phase and provides it to the resonant inverter 707.

In step 1006, the power dosage is calculated and, in step 1008, the power dosage is compared to a requested power dosage. If the power dosage is equal to the requested power dosage, the method 1000 returns to step 1002. If not, voltage and current are measured at the output of the generator circuitry 800, in step 1012. In step 1014, the measured voltage and the measured current are compared to the voltage limit and the current limit, respectively. When the measured voltage and the measured current are less than the voltage limit and the current limit, respectively, the method 1000 returns to step 1002.

In the case where the measured current is greater than or equal to the current limit, the controller 849 generates control signals d₁ and d₂ having different phases from those of already generated two control signals and provides them to the two boost converters so that a current lower than the current limit is generated, in step 1016. In the case where the measured voltage is greater than or equal to the voltage limit, the controller 849 generates a control signal p having a different fixed phase and provides it to the resonant inverter 707 so that a voltage lower than the voltage limit is generated, in step 1018. Then, the method 1000 returns to step 1006.

Although the illustrative embodiments of the present disclosure have been described herein with reference to the accompanying drawings, it is to be understood that the disclosure is not limited to those precise embodiments, and that various other changes and modification may be effected therein by one skilled in the art without departing from the scope or spirit of the disclosure. 

What is claimed is:
 1. An electrosurgical generator comprising: a power converter coupled to an electrical energy source and configured to generate electrosurgical energy, the power converter including: a boost converter configured to convert a first direct current from the electrical energy source to a second direct current; and a phase-shifted pulse width modulation (PS-PWM) resonant inverter configured to invert the second direct current to an alternating current; a plurality of sensors configured to sense voltage and current waveforms of the generated electrosurgical energy; and a controller coupled to the power converter and the plurality of sensors, the controller comprising: a signal processor configured to determine a tissue impedance based on the sensed voltage and current waveforms; and an output controller configured to select one among a plurality of output characteristics based on the tissue impedance, and to generate a first control signal to control the boost converter and a second control signal to control the PS-PWM resonant inverter, according to the selected output characteristic, wherein a switching frequency of the boost converter is greater than a switching frequency of the PS-PWM resonant inverter and the switching frequency of the boost converter is an integer multiple of the switching frequency of the PS-PWM resonant inverter, wherein the plurality of output characteristics include a constant current output characteristic, a constant voltage output characteristic, and a constant power output characteristic, wherein, when the output characteristic is the constant voltage output characteristic, the output controller generates the second control signal having a first fixed phase and a varying duty cycle, and wherein, when the output characteristic is the constant power output characteristic, the output controller generates the second control signal having a second fixed phase, which is different from the first fixed phase, and a fixed duty cycle.
 2. The electrosurgical generator according to claim 1, wherein the output controller shifts from the constant current output characteristic to the constant power output characteristic and from the constant power output characteristic to the constant voltage output characteristic based on the tissue impedance.
 3. The electrosurgical generator according to claim 1, wherein the output controller selects the constant current output characteristic if the tissue impedance is less than a first predetermined value, wherein the output controller selects the constant power output characteristic if the tissue impedance is greater than or equal to the first predetermined value and less than a second predetermined value, wherein the output controller selects the constant voltage output characteristic if the tissue impedance is greater than or equal to the second predetermined value and less than a third predetermined value, and wherein the first predetermined value is less than the second predetermined value and the second predetermined value is less than the third predetermined value.
 4. The electrosurgical generator according to claim 1, wherein, when the output characteristic is the constant voltage output characteristic, the output controller generates the first control signal under a voltage control mode.
 5. The electrosurgical generator according to claim 1, wherein, when the output characteristic is the constant current output characteristic, the output controller generates the first control signal under a voltage control mode and generates the second control signal having a third fixed phase.
 6. The electrosurgical generator according to claim 1, wherein, when the output characteristic is the constant power output characteristic, the output controller generates the first control signal under a current programmed mode.
 7. The electrosurgical generator according to claim 1, wherein, when the output characteristic is any one of the constant voltage output characteristic, the constant current output characteristic, and the constant power output characteristic, the output controller generates the first control signal under a current programmed mode.
 8. The electrosurgical generator according to claim 1, wherein the boost converter includes a plurality of boost converters to lower ripples of the voltage and current input to the PS-PWM resonant inverter.
 9. The electrosurgical generator according to claim 1, wherein the electrosurgical generator further comprises a plurality of analog-to-digital converters (ADCs) configured to sample the sensed voltage and the sensed current to obtain a predetermined number of samples of each of the sensed voltage and the sensed current, and wherein the predetermined number of samples corresponds to an integer multiple of an RF frequency of the sensed voltage and the sensed current.
 10. The electrosurgical generator according to claim 1, wherein the plurality of sensors includes a Rogowski coil.
 11. The electrosurgical generator according to claim 1, wherein the controller is implemented in a field programmable gate array, an application specific integrated circuit, or a digital signal processor.
 12. The electrosurgical generator according to claim 1, wherein the output controller is further configured to select one mode among a plurality of modes based on the output characteristics.
 13. The electrosurgical generator according to claim 12, wherein the plurality of modes include a voltage control mode and a current programmed mode.
 14. The electrosurgical generator according to claim 12, wherein the output controller generates the first control signal based on the selected mode to control the boost converter. 